In association with improvement in a mobile communication technique and the like, the frequency band of waves used for communication is being spread to a high-frequency area such as a GHz band and communication devices used for communication are also being miniaturized. RF modules such as a waveguide and a filter used in communication devices of this kind are also being requested to realize higher frequencies and further miniaturization. A waveguide line as disclosed in Japanese Patent Laid-open No. Hei 6-53711 and a filter using such a waveguide line as disclosed in Japanese Patent Laid-open No. Hei 11-284409 have been developed. As connection structures for connecting an RF module of this kind, connection structures as disclosed in Japanese Patent Laid-open Nos. 2000-216605 and 2003-110307 have been developed.
In this case, the waveguide line disclosed in Japanese Patent Laid-open No. Hei 6-53711 includes, as shown in FIG. 1 in the publication, a dielectric substrate (1) having conductor layers (2 and 3) and a plurality of conduction holes (4) which connect between the conductor layers (2 and 3) and are disposed in two lines. The waveguide line is constructed by a pseudo rectangular waveguide in which a region in the conductor is used as a line for transmitting a signal by surrounding all directions of a dielectric material with the pair of conductor layers (2 and 3) and pseudo conductive walls formed by the plurality of conduction holes (4). In this case, a waveguide line having such a configuration is also called a dielectric waveguide line.
The filter disclosed in Japanese Patent Laid-open No. Hei 11-284409 is constructed by, as shown in FIG. 1 in the publication, disposing a plurality of through conductors (26) forming an inductive window (coupling window) so as to establish electric connection (conduction) between a pair of main conductor layers (22 and 23) in a dielectric waveguide line (25) as a pseudo rectangular waveguide constructed by a dielectric substrate (21), the pair of main conductor layers (22 and 23) and a through conductor group (24) for sidewalls in a similar manner to the waveguide line disclosed in Japanese Patent Laid-open No. Hei 6-53711. Since the filter can be formed inside the dielectric substrate such as a wiring board, the filter can be easily miniaturized.
In a connection structure between a dielectric waveguide line (pseudo rectangular waveguide) and a line conductor (microstrip line) disclosed in the Japanese Patent Laid-open No. 2000-216605, as shown in FIG. 1 in the publication, an end of a line conductor (20) is inserted into an open end of a dielectric waveguide line (16), and the end and one main conductor layer (12) are electrically connected to each other via a line conductor (18) for connection and a through conductor (17) for connection so as to form steps. The connection structure is a so-called ridge waveguide structure in which the interval between the pair of main conductor layers (12 and 13) is narrowed. Therefore, at the time of propagation of RF signals (electromagnetic waves) from the line conductor (20) to the dielectric waveguide line (16), electromagnetic waves propagating in the TEM mode through the line conductor (20) are mode-converted into electromagnetic waves propagating in a TE mode (TE10 mode) through the dielectric waveguide line (16). In other words, the connection structure changes the line conductor (microstrip line) to the waveguide line.
On the other hand, in a connection structure between the waveguide line (in this example, the waveguide line is a component of a dielectric waveguide filter) and a line conductor (microstrip line) disclosed in the Japanese Patent Laid-open No. 2003-110307, as shown in FIG. 1 in the publication, protruding portions (17a and 17b) are formed on the outside of dielectric waveguide resonators (11a and 11d) forming a dielectric waveguide filter, and conductive strip lines (15a and 15b) extending from the bottom surfaces of the dielectric waveguide resonators (11a and 11b) to the protruding portions (17a and 17b) and serving as input and output electrodes are formed. The conductive strip lines (15a and 15b) are connected to conductive patterns (19a and 19b) as line conductors formed on a wiring board (18). In the connection structure, the conductive patterns (19a and 19b) are terminated on the bottom surfaces of the dielectric waveguide resonators (11a and 11d) via the conductive strip lines (15a and 15b) formed so as to have the same width as that of the conductor patterns (19a and 19b). Thus, to the bottom surfaces of the dielectric waveguide resonators (11a and 11d), input and output signals in the TEM mode are supplied via the conductive patterns (19a and 19b), respectively. Therefore, magnetic fields generated in the dielectric waveguide resonators (11a and 11d) by the input and output signals are coupled to magnetic fields in a fundamental resonance mode (TE mode (TE10 mode)) of the dielectric waveguide resonators (11a and 11d). As a result, electromagnetic waves propagating in the TEM mode in the conductive patterns (19a and 19b) are mode-converted into electromagnetic waves propagating in the TE mode (TE10 mode) in the dielectric waveguide resonators (11a and 11d) as dielectric waveguide lines. Electromagnetic waves propagating in the TE mode (TE10 mode) in the dielectric waveguide resonators (11a and 11d) are mode-converted into electromagnetic waves propagating in the TEM mode in the conductive patterns (19a and 19b). That is, the connection structure has the line converting function of converting a line conductor (microstrip line) to a waveguide line or converting a waveguide line to a line conductor.
Incidentally, for example, as disclosed in the Japanese Patent Laid-open Nos. 2000-216605 and 2003-110307, although most of RF modules currently proposed are to output electromagnetic waves in the TEM mode from the dielectric waveguide line (waveguide) as unbalanced electromagnetic waves, there is also a demand for realizing an RF module (mode converter or line converter) which outputs balanced RF signals in the TEM mode from a waveguide through which electromagnetic waves in the TE mode propagate. To address the demand, for example, an RF module (dielectric filter) as disclosed in Japanese Patent Publication No. 3351351 has been proposed. In the dielectric filter, as shown in FIG. 1 in the publication, on an outer surface of a dielectric block (1), an external terminal (8) continued from one end of an external coupling line (25) and an external terminal (6) generating capacitance in cooperation with a resonance line (5a) are formed, thereby constructing an unbalanced to balanced conversion circuit. The phase difference between one of output signals output from the external terminal (6) by the capacitive coupling and the other output signal output from the external terminal (8) by the inductive coupling is set to 180 degrees by adjusting a capacitance value and an inductance value of the coupled portions.
However, the unbalanced to balanced conversion circuit disclosed in the Japanese Patent Publication No. 3351351 has the following problems. In the unbalanced to balanced conversion circuit, in order to set the phase difference between the two output signals to 180 degrees, the capacitance value of the capacitive coupling and the inductance value of the inductive coupling have to be adjusted. Therefore, the unbalanced to balanced conversion circuit has the problems such that it requires some time and effort for the adjustment work and it is difficult to miniaturize the circuit since a signal path which is not operated as a resonator has to be provided in addition to a resonator.